9.2.1 Analog Electrical Potential Control

You can change the continuous voltage across a DC motor by a transistor. Without going into details, this can be done with a N-channel MOSFET with an opamp, or a bipolar junction transistor (BJT) with a variable bias current.

In the past, it is difficult to adjust the continuous voltage from a controller without a digital-to-analog converter (DAC). This is much easier these days because of digitally controled potentiometers. In other words, it is possible to control a DC motor by controling the continuous voltage.

However, this approach has a problem. Power dissipation is $P=VI$. In other words, the power that needs to be dissipated as heat is proportional to the product of the voltage drop across a device and the current passing through the device. Let us consider a modest DC motor that draws up to 4A at 12V. In order to produce 6V across the motor, the other 6V must drop across the transistor (N-MOSFET or NPN BJT). The current consumption at 6V is probably about 2A. This means the power dissipated by the transistor is 12W.

12W does not seem to be much. However, if this much power needs to be disspiated by a small device like a transistor, it is a lot! If you try to control the continuous voltage across a DC motor, you'll need lots of heatsink area and even forced air cooling (use a fan to blow on the heatsink).